It has long been known that the feeding of web material, such as paper or other fragile sheeting, from an input to an output is best accomplished by permitting a loose loop to form in the web between the inlet and the outlet. With such a loop, the web will not be damaged if there are changes in the relative infeed and outfeed speeds of the web. However, if the infeed speed remains greater than the outfeed speed, the loop will continually grow in size. Conversely, if the infeed speed remains less than the outfeed speed, the loop will continually diminish in size, and this may result in the total disappearance of the loop.
Control over the infeed and outfeed speeds of the web in order to maintain a loose loop becomes particularly critical when the web is utilized in a system wherein the tension of the web must be kept uniform. For example in the printing industry, it is common for web-fed rotary printing presses to withdraw the web from a supply roll and draw the web through one or more printing units of the press. In a multiple inking process, the individual printing inks are applied to the web in succession and must therefore be accurately registered. To obtain accurate registration, the web must remain in constant tension while traveling along the length of the press. As is evident, non-uniform changes in the tension of the web during the printing process can directly effect the physical shape of the web, especially when using dampener-operated offset printing presses. Moreover, changes in the web tension can distort the intended printing operation by a printing unit if the change causes slippage between the impression cylinders. As a result, the correct registration of ink during the printing process can become very difficult or impossible.
Means to maintain a web with a desired loose web while operating under uniform tension has long been sought in the printing art, and many proposals have attempted to accomplish this. According to French Pat. No. 426,833, the tension of the web may be regulated by changes in the rotational speed of two rollers which withdraw the web from a supply roll. The changes in speed are controlled by the deflection of a pendulum roll which engages the web with a constant pressure. This simple and primitive method of web tension control can only eliminate disturbances caused by, and during the withdrawal of, the web from the supply roll.
Published German patent application No. 1,060,406 discloses a printing press in which pairs of power-driven conveyor rollers are positioned in front of and behind the printing unit or units of the press. The rotational speed of each pair of rollers is manually and independently adjustable. This German application also refers to means for measuring and controlling the web tension but does not further describe such means. In effect, the press arrangement described in the German application subdivides the web into several aligned but mutually separate portions which are adjusted independently of each other. As a result, there is no assurance that the tension of the web is stable and constant across the entire length of the web passing through the press. Moreover, if the web is to be printed on both sides, it is not possible to provide tension probing devices at various points along the length of the web passing through the press; such devices would smear the freshly printed ink.
U.S. Pat. No. 2,897,754 discloses a device in which the withdrawal speed of the web is controlled by a friction belt drive acting upon the circumference of the web supply roll. The speed of the belt drive is controlled by the interposition of a finely adjustable variable speed gearing, which is controlled by the displacement of a tension roller placed behind the supply roll, acting upon the web as it is withdrawn from the roll. The device also incorporates suitably connected, finely adjustable variable speed gearings which allow for variations in the rotational speeds of pairs of conveyor rollers positioned ahead of the printing units and behind the drying units. The patent is silent as to the factors to be taken into account in determining the rotation speeds of the conveyor rollers placed behind the printing units. There are also no provision for comparing the rotational speed of the conveyor rollers behind the printing units with the rotational speed of the conveyor rollers in front of the printing units, and hence adjustment on the basis of such comparison is not possible.
U.S. Pat. No. 3,561,654 discloses a device for maintaining constant tension in a web of paper being withdrawn from a supply roll and conveyed through one or more printing units of a printing press. The device consists of two pairs of power-driven rollers, one placed in front and the other in back of the printing unit or units, a means for visually measuring digitally indicated speed differentials between the two pairs of rollers, and a potentiometer controlled means for adjusting the speed of the rollers to the desired value. The system is expensive to install and does not react quickly enough for use in modern high-speed printing press operations.
U.S. Pat. No. 3,485,427 discloses a tension controlling device. A complicated electronic circuit is relied upon to control the tension of a web.
U.S. Pat. No. 3,138,168 discloses a reverse acting, pneumatic amplifier which is generally structurally similar to the transducer-amplifier utilized in the loose loop feed control apparatus of the present invention. However, the apparent novel feature of this patented amplifier resides in the use of a single diaphram and an integral flapper combination as the only active element of the amplifier. This patent does not disclose nor teach the use of the patented amplifier in a loose loop feed control apparatus.
In the past, it was also known to use devices to control web tension between pairs of rollers by means of electrical pulses generated as a function of the rotational speeds of the rollers. The pulses are counted by a suitable counter and digitally totaled per unit of time. The totals obtained are used to measure rotation ratios and slippage. The relative rotational speeds of the roller pairs are then adjusted either by automatic or manual controls.
It is an object of the present invention to provide an improved apparatus which is utilized to control the infeed speed of a continuous web so that a loose loop is formed and maintained in the web so as to substantially eliminate damage to the web.
Another object of the present invention to provide an improved loose loop feed control apparatus wherein a vacuum signal, which varies as the length of the descending portion of the web loop increases or decreases, operates, through a transducer-amplifier, a ratio changing device, such as a variable pitch sheave, to automatically change the web's infeed speed to match the web's outfeed speed, without the necessity of manually adjusting the speeds or providing complicated electronic means to do so.
A still further object of the present invention is to provide an improved loose loop feed control apparatus of the type described wherein because of the usage of the apparatus, the direction of the web may be changed through a substantial angle, for example, 90.degree. or even 180.degree. degrees, and wherein the web can be turned upside down, i.e., the top and bottom sides or surfaces of the web can be reversed, without substantial risk of damage to the web.